Sheets which are conveyed rhythmically one after another with respect to their rear edges are produced, for example, by rotational cutters. Using a known cutter which cooperates with a device for stacking, the sheets pass between high-speed upper and lower conveyor belts after having been cut. A low-speed conveyor belt is arranged in a conveying direction behind the lower high-speed conveyor belt, and the upper side of the low-speed conveyor belt is displaced downwards opposite a joint conveying plane of the high- and low-speed lower and upper conveyor belts. A deflecting member is arranged above the conveying plane in the transitional zone between the high- and low-speed lower conveyor belts. The deflecting member is controlled rhythmically with the on-coming rear edges of the sheets, and thus deflects the rear edges of the sheets downwards from the conveying plane onto the lower slower conveyor belt. The downward deflection is carried out in order to obtain space for the leading edge of the next sheet, which is to be separated from the previous sheet. Control of the compressed air by the deflecting member, however, causes difficulties as it is impossible, especially upon a quick succession of sheets (the sensing elements, the relay control, valve, and the air all need more time than the interval between the sheets allows) to keep the compressed air, which deflects the sheet, away from the leading edge of the next sheet. A clean separation is therefore not possible, at least with sheets which follow each other in quick succession. However, when overlapping occurs, the overlapped sheets which have been deposited on the lower low-speed conveyor belt are further transported to the stacking point. Further separation only takes place to a limited extent on this stretch. This means that the front edge of the sheet collides with a stop of the stack at a considerably high speed, dependent on the speed at which the sheets are fed and the degree of overlapping. In the case of thin sheets having a large format, such a collision may lead to crushing of the sheet, setting up a relationship between the stiffness of the sheet and the kinetic energy of the sheet which is unfavorable. (German Offenlegungsschrift No. 1,245,702).
In a different type of separation device, which does not have pneumatic conveyor and braking means, but operates with rollers which come into contact with the material to be conveyed, a pair of rollers is provided as braking means. One of the rollers has a projection, and the other of the rollers has a recess which rotates in synchronization with the former. Whilst one sheet entering the roller gap is not affected by the other area of the two rollers, the sheet end is affected by the projection and the recess. The sheet can consequently be conveyed without any interference by the pair of rollers only until its end, and is then affected by the projection and the recess. As the speed of rotation of the pair of rollers is less than the speed at which the sheet is conveyed, the sheet is decelerated. As the periphery of the projection and the recess are downwardly displaced opposite the conveying plane, the end of the sheet is also deflected downwards at the same time as the braking occurs, so that the leading edge of the next sheet, which is conveyed at a higher speed than the decelerated sheet, can be dropped on top of the decelerated sheet.
In this known device the end of the sheet is indeed also deflected downwards from its conveyance plane by means of the projection; however, this deflection merely serves for sheet separation; no transfer into the effective area of the braking means occurs. The conveyor means remain fully effective. (German Auslegeschrift No. 2,032,800).